1. Field of the Invention
The present invention relates to a wireless transmission system, a wireless transmission method, a wireless reception method, a transmitting apparatus and a receiving apparatus.
2. Description of the Related Art
With the spread of a local area network (LAN) owing to computerization in recent years, a demand for a wireless LAN has risen owing to the problems of wiring construction in an office, the shortening of a construction period, a problem concerning an interior fine view, the complexity of maintenance, operation and management, and the like. As a wireless transmission system to be used for such a wireless LAN, the ultra wide band (UWB) wireless transmission system is known.
The ultra wide band wireless transmission system basically performs baseband transmission by the use of a signal composed of a pulse train having a very fine pulse width (for example, one nanosecond (ns) or shorter). The UWB wireless transmission system forms spread information by multiplying a prescribed wireless signal, for example transmission information, by a prescribed spread code sequence. Moreover, the UWB wireless transmission system generates a short impulse at the period of several hundreds nanoseconds, and uses a signal generated by changing the phase of the impulse or the time change of the impulse in accordance with the above-mentioned spread information as a transmission signal. On the other hand, an apparatus for receiving information discriminates an information bit of the impulse signal by means of the above-mentioned phase or the fine time change of the transmitted impulse. The apparatus for receiving information obtains a desired information bit by performing the reverse spread of the transmission signal by the use of a prescribed spread code sequence. Moreover, the occupied bandwidth of the UWB wireless transmission system is a bandwidth of the order of a gigahertz (GHz) such that a value obtained by dividing the occupied bandwidth by the center frequency thereof (for example, being within a range from 1 GHz to 10 GHz) becomes about 1. The bandwidth thereof is an ultra wideband in comparison with the bandwidths used in the so-called wideband code division multiple access (W-CDMA) system, the cdma2000 system and a wireless LAN using a spread spectrum (SS) or the orthogonal frequency division multiplex (OFDM).
Moreover, the ultra wide band transmission system has the feature in which the existing systems having a high signal power density characteristic at a specific frequency band is not easily subjected to interference owing to the low signal power density characteristic of the UWB system. Consequently, the UWB system is expected as a technique capable of overlaying the frequency bands used by the existing wireless systems. Moreover, because the USB system has a wide band, a hopeful view is taken on the USB system as a ultra high speed wireless transmission technique at a 100 Mbps level for the use of a personal area network (PAN).
On the other hand, if the case where two or more UWB wireless networks being uncoordinated with each other are existed in the same area is supposed in the UWB wireless transmission, it can be considered that the UWB wireless transmission may give large interference to each transmitter and receiver depending on their positional relationships. In such a case, because the UWB wireless transmission uses an ultra wideband in occupied bandwidth, there are no frequency channels for the escape of the interference. In the worst case, there is the apprehension that it becomes impossible to perform communication. Here, the expression “uncoordinated to each other” means that channel assignment information and the like are not shared by control stations that control individual network.
As one of means for solving the problem, the time division multiple access (TDMA) system is used in which a channel is divided into frames and resources are assigned to each frame.
The time division multiple access system performs continuous resource assignment over a comparatively long time in a frame to a single piece of communication in a network.
The conventional TDMA system adopts the following frame structure. An example of the frame structure is shown in FIGS. 13A and 13B.
In the TDMA system, as shown in FIG. 13A, for example, unit frames 1301, 1302 and 1303 of the TDMA system (called as “TDMA frames”) are repeated. The length of each of the TDMA frames 1301, 1302 and 1303 is, for example, one microsecond.
In each of the TDMA frames 1301, 1302 and 1303, as shown in FIG. 13B, a beacon 1304 being an identifying signal including the assignment information of wireless resources (resource assignment information) is arranged at the head of the frame. Next to the beacon 1304, areas for terminal station (or users) included in the wireless network are assigned. In the example shown in FIG. 13B, areas (referred to as “user assignment areas”) 1305, 1306 and 1307 which are assigned for a terminal station-A, a terminal station-B and a terminal station-C, respectively, are set in the order after the beacon 1304. The areas 1305, 1306 and 1307 which are assigned for each of the terminal stations-A, -B and -C may be variable in each frame.
Moreover, in an area other than the beacon 1304 and each of the user assignment areas 1305, 1306 and 1307, a contention period 1308 is set. The contention period 1308 is an area to be used for as a random access channel from a terminal station to a base station or communication between terminal stations. Because the contention period 1308 is not an interval assigned by the base station, contention of communication in the network can occur in the contention period 1308.
In communication using such a TDMA frame, for example, a terminal station demands resource assignment (a transmission demand) at the next frame in a random access channel (RACH) in the contention period 1308. Then, the base station determines user assignment areas for the resource assignment in the next frame in response to the demand. Furthermore, the base station informs the determination to each of the terminal stations-A, -B and -C by means of a beacon 1309 in the next frame. Then, each of the terminal stations-A, -B and -C performs communication on the basis of the resource assignment information of the beacon 1309.
If two or more uncoordinated UWB networks performing communication using the TDMA frames are arranged closely to each other, the interference to stations in the network easily occur continuously. In such a case, there is the problem in which the station having received the interference cannot restore data by error correction or the like, and the communication becomes impossible.
FIG. 14 shows a state in which two networks are arranged closely to each other. As shown in FIG. 14, a personal area network (hereinafter referred to as a “PAN”)-X 1401 and a PAN-Y 1402 are arranged closely to each other in an uncoordinated state to each other. The PAN-X 1401 is composed of a base station-X 1403, a terminal station-A 1405, a terminal station-B 1406, a terminal station-C 1407 and a terminal station-F 1410, wherein all of the terminal stations-A 1405, -B 1406, -C 1407 and -F 1410 are controlled by the base station-X 1403. On the other hand, the PAN-Y 1402 is composed of a base station-Y 1404, a terminal station-D 1408 and a terminal station-E 1409, wherein both of the terminal stations-D 1408 and -E 1409 are controlled by the base station-Y 1404.
Moreover, the terminal station-C 1407 and the terminal station-E 1409 are at a positional relationship in which, if either of the terminal stations-C 1407 and -E 1409 performs wireless transmission, the wireless transmission interferes with a wireless signal received by the other.
FIGS. 15A and 15B show the states of the frames of the PAN-X 1401 and the PAN-Y 1402, respectively. FIG. 15A shows the state of a frame of the PAN-X 1401 at a certain point of time, and FIG. 15B shows a frame of the PAN-Y 1402 at the same point of time.
As shown in FIGS. 15A and 15B, a user assignment area 1501 assigned to the terminal station-F 1410 (the communication from the terminal station-F 1410 to the terminal station-C 1407 is supposed) and a user assignment area 1502 assigned for the transmission of the terminal station-E 1409 are in the state of overlapping in time with each other. As shown in FIGS. 15A and 15B, in the case where the user assignment areas to which the terminal station-C 1407 belonging to the PAN-X 1401 and the terminal station-E 1409 belonging to another PAN-Y 1402, both of the terminal stations-C 1407 and -E 1409 being located closely to each other, are assigned are in contention with each other, it is apprehended that the communication between the terminal stations-F 1410 and -C 1407 becomes impossible.
Consequently, it was necessary to impose some limitations on using communication apparatus constituting each network for avoiding the above-mentioned situation. For example, it was necessary to avoid such a situation in which two or more networks being uncoordinated to each other are located in the same area.